The present invention relates to a dot matrix printing head and a printing apparatus using such head.
It is known that serial impact dot matrix printers are broadly used as computer peripherals by reason of their reliability, low cost, versatility and high printing speed.
When suitably driven by printing control programs they can virtually print any kind of fonts, as well as diagrams and drawings, on several kind of printing supports, from thin paper sheets to multiple copy forms.
The achievable printing speed is very high. The same equipment may achieve a speed of 600 characters per second when printing in draft mode, and a speed of 200 characters per second when printing in near letter quality mode.
The factor which limits the printing speed is essentially the maximum needle operative frequency which, in turn, is determined by the time required to energize an electromagnet, driving the needle against the printing support, and by the time required for the needle to return in a rest position, after it has impacted on the printing medium.
Today operative frequencies in the range of 2000-2500 Hz can be attained, which however impose some restriction as to the operative conditions of the printing device.
The shorter is the stroke requested to the needles and the lesser is the energy which the needle has to transfer by impact to the printing medium, the higher is the operative frequency.
A short stroke and a low impact energy impose a limit to the thickness of the printing medium and the number of copies which may be printed, which may be overcome, partially, by the precise machining of the printing apparatus and its set up.
The printing apparatuses of today have therefore to set a compromise between the operative printing speed and the characteristics of the printing medium.
If the printer has to print on multiple copy paper, the print head must be designed and set up to this purpose, achieves a maximum operative frequency related thereto and even when printing on single sheets the maximum operative frequency remains the same.
This problem is overcome by the dot matrix printing head of the present invention which is provided with means for dinamycally changing its set up on demand, so as to establish the most suitable operative conditions as a function of the characteristics of the printing support.
The stroke of the needle, in a print head of today, where the needle is driven by an end of an electromagnet armature, acting as a lever actuated by electromagnet energization, is determined by the width of the air gap existing between the electromagnet and the armature at rest.
The gap width is set up in the manufacturing process.
According to the invention means are provided to dynamically change the gap width on operator command, so as to set it at best for the requested operative purpose.